Composite materials are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or microscopic scale within the finished structure. There are two categories of constituent materials: matrix and reinforcement. At least one portion of each type is required. The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart their special mechanical and physical properties to enhance the matrix properties. A synergism produces material properties unavailable from the individual constituent materials, while the wide variety of matrix and strengthening materials allows the designer of the product or structure to choose an optimum combination.
Commercially produced composites often use a polymer matrix material often called a resin solution. There are many different polymers available depending upon the starting raw ingredients which may be placed into several broad categories, each with numerous variations. Examples of the most common categories for categorizing polymers include polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide, polypropylene, PEEK, and others.
The reinforcement materials for composites are typically fibers but also may be commonly ground minerals. Fiber-reinforced composite materials can be divided into two main categories normally referred to as short fiber-reinforced materials and continuous fiber-reinforced materials. Continuous reinforced materials often constitute a layered or laminated structure. The woven and continuous fiber styles are typically available in a variety of forms, being pre-impregnated with the given matrix (resin), dry, uni-directional tapes of various widths, plain weave, harness satins, braided, and stitched. Various methods have been developed to reduce the resin content of the composite material, by increasing the fiber content. Typically, composite materials may have a ratio that ranges from 60% resin and 40% fiber to a composite with 40% resin and 60% fiber content. The strength of a product formed with composites is greatly dependent on the ratio of resin to reinforcement material.
Pre-preg is the general term used for “pre-impregnated” composite fibers where a material, such as epoxy is already present. As described above, pre-preg fibers usually have of a weave or are uni-directional. Pre-preg materials already contain an amount of the matrix material used to bond the pre-impregnated fibers together and to other components during manufacture. Pre-preg materials are mostly stored in cooled areas since activation is most commonly done by heat. Hence, composite structures built of pre-pregs typically require an oven or autoclave to cure out.
Pre-preg compression molding utilizes pre-preg materials, which are typically made from continuous fiber reinforced materials, that have been cut into sheets or plies with specific patterns. During the compression molding process, the pre-preg plies are hand assembled into a preform made up of multiple layers of the pre-preg ply material, and then the preform is placed into a mold and compressed, which consolidates the layers of pre-preg material and cures the pre-preg. However, since pre-preg material typically has a continuous fiber format, the pre-preg material typically will not flow in the mold to any significant degree. Consequently, when there are irregular thicknesses or features that standout from the surface, such as a fastener boss (as best seen in FIG. 5), that require a depression or void in the mold to be filled it is helpful to add a material that will flow to either the pre-preg as a separate layer or to the preform as a separate material located adjacent to the irregular geometry that needs to be filled. However, even with adding extra material, the problems associated with getting material to flow into irregular geometries still persist. In the case of bosses into which an insert will be installed either as part of the molding process or post molding, it is difficult to get these bosses to completely fill without porosity, or material voids. The existence of porosity in a molded structure can create leak paths. In addition, post molding assembly of inserts such as fasteners or threaded channels into a molded boss that has poor flow can result in brittle material. The brittle features of the bosses and other securement features may lead to premature failures of compression molded assemblies, and potentially dire consequences for a user depending on the strength and reliability of the molded assembly.
Thus, there exists a need for devices and methods that form more reliable and robust surface features that are free of material voiding, where the surface features standout above a compression molded surface including surface features such as bosses that accommodate fastener inserts.